This invention applies in particular, without being limited thereby, to the drying of biological sludge from urban and/or industrial wastewater purifying stations.
The present invention is vitally important at a time when, throughout the world, research and applications are being developed on techniques which serve to conserve fossil energy resources and to control global warming due to the greenhouse effect.
A person skilled in the art knows that purification sludge is an inevitable waste produced by water treatment: every individual produces an average of 20 kilograms per year thereof, representing more than 10 million metric tons per year for Europe, expressed as dry matter, representing a 5 times larger figure when applied to so-called wet sludge, which consists of 20% dry matter and 80% water.
At the present time, this sludge can be disposed of in several ways, in particular: dumping, incineration and agricultural spreading. However, owing to numerous constraints (in particular technical, health, regulatory) these sludge disposal systems are increasingly complex, hence increasingly expensive, and even liable to criticism and indeed prohibited locally.
At all events, drying the sludge is an unavoidable step in the wastewater treatment system, because it serves to reduce the volumes to be stored by a factor of 4, facilitating transport and disposal. This drying clearly represents a rapidly growing market today.
The prior art in this field can be summarized as follows:                Thermal drying: this technique uses conventional energy (gas, oil, electricity) as well as specific equipment to transfer energy to the sludge and to evaporate the water present therein. The application of this technique requires large-scale installations, incurring large investments and consuming colossal amounts of energy (1 megawatt per metric ton of water evaporated) which also generate highly polluted condensates which must then be retreated at considerable expense.        Composting: this natural, long-established method, using thermophilic fermentation of the carbonaceous organic matter in the presence of air, causes a heat release that permits partial dehydration of the water present in the sludge. This is a simple method, easy to apply, economical, and capable of yielding a high-grade product. However, it is ineffective for reducing the volume of the sludge, because the drying is only partial and requires the addition of structuring co-products (bark, sawdust, green waste, etc).        Solar drying: this technique is starting to be developed in the field of sludge drying because it is suitable for building simple, economical and ecological installations. However, it presents the major drawback of requiring large greenhouse areas, incurring very large and hence costly investments, as well as problems of layout, thus being limited to rural areas and low-capacity stations.        
So far, it has not been possible, on an industrial scale, to build drying installations that make use of wind energy. This is because the generation of energy by wind generator to supply a conventional sludge drying installation is uncompetitive. Furthermore, hybrid solutions (gas compression, heat pumps) have limited capacity and are only feasible for very small installations. A further drawback of wind energy is that it is not constantly available, therefore requiring oversized drying equipment (by a factor of at least 2), which would lead to uncompetitive solutions.